1. Field of the Invention
This invention relates to disk drives for storing information in computer systems, and in particular to a servo system for a floppy disk drive which enables the storage of increased amounts of information on conventionally available floppy disks.
2. Description of the Prior Art
Floppy disk drives in compute: systems operate by writing data onto a magnetic media, and then reading the data from the media, as desired by the user. The trend in floppy disk drives has been toward smaller and smaller disks, with the 51/4-inch disks of only a few years ago now being commonly replaced by 31/4-inch disks, and with 2-inch disks already in use. As disk sizes have become smaller, computer system users have sought to store increased amounts of information on the disk. The storage of larger and larger amounts of information on the disk requires greater positional accuracy for the read/write head than ever before.
Prior art techniques for achieving the necessary positioning accuracy in a removable medium disk drive storage apparatus have consisted of both open loop servo systems and servo systems with active position feedback, commonly referred to as closed loop servo systems. Removable medium disk drives typically consist of electromechanical systems, where a disk housed in a plastic shell is insertable in the disk drive apparatus. The drive mechanism causes this disk to be seated onto a spindle motor, which then rotates the disk at a specified rpm. Further, the mechanism in the drive positions a read and write transducer at various positions on the disk surface describing concentric tracks. These tracks are centered at a specific point which may not coincide with the center of rotation of the spindle motor, and thus will have an eccentricity due to mispositioning of the insertable medium with reference to the centerline of the spindle motor. Locating these concentric tracks so that new information can be recorded onto the tracks or read back from the tracks is accomplished by a servomechanism. The specific requirements for this servomechanism is to identify the required track on the disk surface, move the read/write transducer to this position, and then maintain the transducer over this area so that the recorded information is directly underneath the read/write gap. A further requirement for such mechanisms is that the read/write gap must be accurately positioned in relationship to the centerline of the concentric track, so that only the required information is available to the head. All adjacent information must be appropriately distant from the gap, thereby having minimal influence on the reliability of the data recorded or read back from the medium.
Prior art techniques for removable disk reading and writing include placing prerecorded servo information on the specific track itself, as shown in FIG. 1. This technique is becoming increasingly popular, because the servo information is resident on the same track where data is being recorded or retrieved. The disadvantage, however, is that the servo information occupies space along with the user data, and thus an "overhead" must be considered. Specific examples are short bursts of servo information followed by user data as shown in FIG. 1. The servomechanism thus can only sample the data at specific intervals around the concentric track. Consequently, the mechanism can react only to those external disturbances detected by this specific sampling frequency. Thus, these systems have a limitation in bandwidth, namely, how rapidly they can compensate for disturbances compared with the dedicated technique.
In another prior art approach a disk surface is dedicated to having only continuous servo information recorded on it over the entire surface, on concentric tracks. A read transducer is continually sampling this information and reacts rapidly to all disturbances that might cause the servomechanism to misposition. The disadvantage of such a technique is that an entire disk surface, which could be used to store user data, is now used by the servo feedback information. Thus, these techniques have only been used in disk drive devices that consist of a multiplicity of disks, termed "disk packs" in the industry.
Both of the above techniques--servo information embedded on the track or on an entire disk surface--require a specialized mechanism that can record the necessary servo information on the disk surface. The equipment used to accomplish this task is called a servowriter in the industry. Servowriters are specialized devices similar to the disk drive mechanism except they are designed with actuator mechanisms with high positional accuracy. This higher positional accuracy is provided over the entire stroke on the disk surface. Consequently, servowriters are slow and require considerable time to generate the servo tracks on a disk medium. Multiple disk devices or disk packs also require a specialized spindle motor, normally with air bearings, so the off-balance load due to the disk pack does not cause the assembly to precess about an axis and thereby develop oblong and inaccurate servo tracks.
Open loop techniques are also in use today, mainly in floppy disk drives. These techniques do not depend upon feedback information from the disk medium, but rely entirely upon the positioning mechanism to locate the read/write transducer to the specified track. This is accomplished by having an actuator mechanism that is an incremental positioning device and a set of erase only transducer elements on either side of the main read/write transducer. When such a device writes a track, the erase elements create a guard band of no data around the written track, as shown in FIG. 2. When these mechanisms position the read/write transducer to retrieve information from the track, the guard band allows limited mispositioning prior to the read/write transducer coming within the vicinity of the adjacent tracks. Such mechanisms are lower in cost and simpler to design. Unfortunately, the guard bands take up significant area and thereby provide a lower track packing density, and consequently result in a lower storage capacity per disk medium. Additionally, the incremental actuator is slow and thus the overall accessing performance is poor.